The present invention relates to a system for injecting fuel into the intake pipes of an internal combustion engine. The system has an air channel branching off from the intake line of the engine ahead of an arbitrarily adjustable throttle flap. A fuel metering device supplies fuel to the air channel in accordance with the engine requirements. A fuel delivery pump receives the fuel and air through a constriction or narrowed cross-section in the channel and passes a fuel-air mixture, via a fuel distributor, to the individual intake pipes of the engine.
A fuel injection system of this type is disclosed in the commonly-owned U.S. patent application Ser. No. 932,493, filed Aug. 10, 1978, of Klaus Emmenthal et al. entitled "FUEL INJECTION APPARATUS". This system is an improvement over the fuel injection system disclosed in the West German Pat. No. 1,243,917. The essential difference between the system in accordance with the U.S. patent application and system disclosed in the German patent is that in the improved system the fuel delivery pump supplies air as well as fuel even under conditions of maximum engine load, whereas in the system disclosed in the patent the pump is designed to deliver substantially pure fuel under conditions of maximum engine load. Only when the engine is operating under conditions of less than maximum power does the fuel pump deliver a mixture of air and fuel.
With a fuel injection system of the type disclosed in the West German Pat. No. 1,243,917 there are therefore operating conditions of the internal combustion engine in which the fuel pump delivers relatively little air. In this case, there is a danger that fuel and air are not delivered continuously but, rather, that "air pockets" are formed which are forwarded to the fuel distributor following the fuel pump. These air pockets are not removed through mixture formation in the distributor so that, as a result, practically pure air is supplied to some of the engine cylinders during some of the piston strokes.
With the design of the fuel delivery pump in accordance with the improvement disclosed in the aforementioned patent application, air is passed through the pump even under conditions of maximum engine output so that the proper mixture formation is ensured. In particular, the fuel is "prepared" already at the fuel pump and also, in any event, as it passes through the comparatively long line or tube between the fuel pump and the fuel distributor. Ideally, the quantity of the auxiliary air in relation to the quantity of fuel is sufficiently large that a film of fuel is deposited on the walls of the fuel line between the pump and the distributor and is rapidly transported by the auxiliary air. This rapid transport is of special importance in the case of engine load fluctuations because the fuel supplied to the engine must be quickly responsive to the load conditions. Accordingly, a homogeneous fuel-air mixture is supplied via the distributor to the intake pipes of the engine. This mixture is extremely rich, however, so that pure air must still be delivered through the intake pipes to the cylinders for proper combustion.
In a fuel injection system of the type disclosed in the aforementioned U.S. patent application a very large fuel delivery pump is required to pump auxiliary air of sufficient quantity, relative to the fuel quantity, that no air pockets occur in the line leading from the fuel pump to the distributor. Particularly in cases of high-power engines, the size of the fuel delivery pump must be very large indeed, although at small and normal loads only a small proportion of the pump capacity is utilized.